The present invention relates to the general field of passive acoustic treatment dedicated to reducing the noise emitted by an aircraft turbojet.
In the field of aviation, international and local regulations are becoming ever more severe concerning the sound nuisance produced by airplanes, and in particular the nuisance that can be attributed to turbojets. Engine manufacturers are thus being called on to reduce the noise from their turbojets, and in particular the fan noise that becomes the major component in the overall noise emitted by a turbojet.
Some reduction in the fan noise of the turbojet can be obtained by modifying the shape and the section stacks of the fan blades. Nevertheless, having recourse to passive acoustic treatment panels remains the main element capable of attenuating such noise. Such panels are usually arranged in the engine nacelle, e.g. downstream from the fan around the outer surface of the primary cowl and the inner surface of the secondary cowl. Such panels are generally made up of one or more layers, each formed by a core of cellular structure sandwiched between a solid skin and a perforated skin.
Furthermore, the present trend is to move turbojet designs towards engines having ever-greater bypass ratios so as to increase the thrust efficiency of such turbojets. Unfortunately, that trend leads to reducing the size of the nacelle, and thus to reducing the available area that can be treated acoustically with passive acoustic treatment panels. Having recourse to two-layer panels enables this reduction in the treated surface area to be compensated, in part. Nevertheless, such a solution is not fully satisfactory in terms of reducing sound emission.